Trouble adjusting GY6 valve lash
 

While trying to diagnose a low power at high rpm on my Hammerhead Twister with a 150cc GY6, I tried a to adjust the valve lash yesterday.

I pulled the covers, turned the crank to top dead center (flywheel at the "T" mark with both cam lobes pointed generally away from me), and found that there was zero valve lash on either valve. None. Nada. Zippo. Yes, I'm sure this was at TDC. Um...

I loosened them both up to .004" (intake) and .005" (exhaust), then turned the crank to test it. The intake valve seemed to stay at about that same setting throughout the back half of its cam lobe. However, as I turned the crank another 120 degrees or so (60 degrees of cam rotation), to where the exhaust lobe was 180 from its peak, the exhaust valve lash got a whole lot bigger. Like 0.020" or more. It's as if the exhaust lobe hadn't quite bottomed out yet at TDC. Is the exhaust duration really that wide? I hadn't checked the lash at any point other than TDC before I started, so I don't know what the lash may have been set to at this point.

So what do I do? Every tutorial I can find online says to set the lash at TDC, not at the back side of each respective cam lobe. However, it makes me nervous to let the valve stay set with 0.020" or more of lash at its low point.

I don't understand part of your description.
I'm wondering if you were on the "wrong" TDC. Every other TDC will work, every other TDC won't work.

Most GY6 cam sprockets have three holes. The two small holes should be positioned parallel to the edge of the head. The third larger hole should be above the edge of the head. If the large hole is below the edge then your on the wrong TDC and need to turn the crank one revolution.

The cam lobes take up half (180') of the cam so any where along the opposite half the cam flat will give you the same gap. You should be able to see if your on the lobe or not.


If that's all good then the cam chain may be out of time.